Background: Guidelines recommend adults should engage in moderate exercise, such as walking, for at least 150 minutes per week in episodes of at least 10 minutes duration. A typical adult can reach this threshold by walking 7,000 steps per day. Unfortunately, only 5% of adults in the United States meet these requirements, and more than 1 in 3 Veterans over age 45 meet the definition of physically inactive. A patient incentive program for physical activity (PA) may help. Behavioral economics suggests that our chronic inability to start and maintain a PA routine may be the result of ?present bias,? which is a tendency to value small immediate rewards over large rewards in the distant future. For many people, the immediate gratification of a sedentary activity, such as watching television or surfing the internet, is a more powerful motivator than the intangible satisfaction of a physically active lifestyle. Patient incentives may overcome present bias by moving the rewards for healthy behaviors forward in time. In a patient incentive program, patients are given tangible, timely rewards for achieving specific health goals, such as walking 7,000 steps per day. Significance/Impact: Regular physical activity (PA) is associated with reduced mortality and lower risks of many diseases, including type 2 diabetes, heart disease, and depression. this study addresses the chronic conditions HSR&D priority area. Innovation: There is little evidence on what type of incentive works best for Veterans. We propose to study incentives for PA in a novel form of randomized trial?A Multiphase Optimization STrategy (MOST) trial. Our objectives are to determine the optimal design of a 12-week patient incentive program to encourage walking among physically inactive Veterans age 50-70. The primary outcomes are optimized components of the intervention, which will be tested against a usual care control group in a future, separate randomized trial. Specific Aims: Aim 1: Conduct a 24 factorial designed screening-phase trial of incentives for increasing average steps per day to 7,000 steps over 12 weeks among physically inactive Veterans. We will test four different incentive factors: 1) lottery vs. loss framed incentives, 2) financial vs. non-financial incentives, 3) a pre- commitment postcard reminder of a Veteran?s stated intrinsic reason for commitment to PA vs. no pre- commitment postcard, and 4) a request for PA advice from a Veteran on staying active vs. no request. The primary outcome is change in steps per week from baseline to week 24. Aim 2. Conduct cost analyses and qualitative interviews. The cost of administering each component and qualitative assessments of the acceptability of each component to trial participants will inform the decision of which components to retain for the subsequent refining and confirmatory phase trials. Aim 3. Convene an expert panel to choose components for the next phases of the MOST trial. The panel will weigh each component in terms of its effect on step counts (Aim 1), administrative costs and participant- reported qualitative assessments (Aim 2), and the strength of the theoretical basis for the component?s effect on physical activity. Methodology: We will enroll physically inactive Veterans age 50-70 in the screening phase trial. The intervention is the four different incentive factors described above. The primary outcome is change in steps per week from baseline to week 24. Implementation/Next Steps: The components derived from this screening phase will be used in a refining phase trial that establishes the optimal dose (frequency, duration, and amount) of the incentive. The optimized intervention will then be tested against a usual care control group in the confirming phase trial. The refining and confirming phase trials will be proposed in a separate, future grant submission.